The present invention relates to a differential pressure sensor which is suitably used for measuring the differential pressure between two fluids, and particularly but not exclusively relates to a multi-function differential pressure sensor which is capable of measuring a static pressure and a temperature as well as the differential pressure.
In conventional differential pressure sensors, there have been known various kinds of multi-function differential pressure sensors which have differential pressure, static pressure and temperature sensors arranged in one chip for simultaneously detecting a differential pressure, a static pressure and a temperature. Earlier attempts are disclosed in Japanese unexamined patent publication (Kokai Koho) Nos. 61(1986)-240134 and 2(1990)-9704, for example. In either attempt, a differential pressure sensor unit as the main sensor is provided on a thin wall, called a pressure sensitive diaphragm, thereof with four semiconductor resistors which are sensitive to a differential pressure. The differential pressure sensor unit is, moreover, provided on a thick wall portion thereof with several semiconductor resistors which are sensitive to static pressure (line pressure) and temperature. These semiconductor resistors are simultaneously formed in the substrate of the semiconductor according to thermal diffusion or ion implantation of the semiconductor production process. The semiconductor substrate is mounted to a stationary base, which is attached to a housing.
In such a kind of multi-function differential pressure sensor, a zero point shift which is caused by changes in line pressure in a process and in temperature is positively compensated for by using signals of an auxiliary sensor, such as static pressure sensor and temperature sensor, mounted on the sensor for producing high accuracy differential signals.
In the earlier attempts, particularly the attempt of Japanese unexamined patent publication 2-9704, bending strains which are caused by the difference in Young's modulus between the semiconductor substrate and the stationary base in application of a static pressure are positively used. Outputs of the static pressure signals are therefore very small, and only compensation signals which are low in S/N ratio can be produced. Moreover, bending strains are generated to produce static pressure signals, and therefore provide influences to the pressure sensitive diaphragm of the differential pressure sensor, so that differential pressure signals and static pressure signals are interfered with each other. For this reason, to obtain a high accuracy differential pressure signals it is necessary to finely collect input and output characteristics of the differential pressure sensor while the temperature and the static pressure are changed.
In Japanese unexamined patent publication 61-240134, differential pressure and static pressure which are to be detected are detected at respective pressure sensitive portions, and static pressure signals are therefore large as compared to those signals in Japanese unexamined patent publication 2-9704. In Japanese unexamined patent publication 61-240134, to produce signals having a high S/N ratio it is however necessary to provide an induction tube portion for introducing a reference pressure to the rear surface of the pressure sensitive portion for static pressure sensor signals. Such a construction is basically much the same as provision of a new static pressure sensor. Thus, the construction and fabrication of the sensor becomes complicated and the sensor deteriorates in reliability and economy. It is preferable to mount various sensors on a single chip as in Japanese unexamined patent publication 2-9704.
In the multi-function differential pressure sensor of either attempt, attention is mainly paid to the zero point shift during application of static pressure to the differential pressure sensor which is the main strain sensor, and the zero point shift is compensated for using output signals from the static pressure sensor as a parameter. In application of a static pressure to the differential pressure sensor, not only the zero point shift but also a change in span is necessarily produced. The compensation of the span change is very hard to accomplish in the collecting of compensation data as compared to compensation of the zero point shift. This is because a predetermined differential pressure must be produced under application of a static pressure to collect the characteristic of the differential pressure sensor. For this reason, regarding a change in span, no compensation is made or simple compensation is carried out. In the differential pressure sensor as the main strain sensor, from the point of measurement accuracy the change of the span should be regarded as important as compared to the shift of the zero point, but in the earlier attempts previously described the span change is not sufficiently compensated.